HTC One M8 doesn’t start, orange light when pluged in

Hy! I have a problem with my M8. The phone doesn’t start anymore, and when I plug it in, the charging-LED turns orange and stays on as long as it charges.

I have tried all button combinations, but without receiving any feedback.

I am supposing my motherboard is dead. How can I test it, as my phone doesn’t give me any feedback? What should I test? I have a multimeter at home, can that help me with figuring out if the motherboard is damaged or not? If yes, how? What should I do? Which connections should I test?

I don’t wannt to buy a new one and then to figure out that not the mainboard was actually the problem. Please HELP!!! :confused::confused::confused: